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Are Semicolons Necessary in JavaScript?

21 points by jashkenas 16y ago

deleted 16y ago
[deleted]
selectnull 16y ago
To the author of the website: you have modified google analytics code and removed semicolons, I congratulate you on that. It shows true conviction. :) Joke aside, that's a good content put together in very clear and concise manner.
jpadvo 16y ago
I always use semicolons to end lines in JavaScript because it is comfortable for me. ...and I always feel a little guilty about it, because it seems wrong to add superfluous characters to my code. Is it actually "personal preference" and I've been feeling guilty for nothing, or is it better to leave off the semicolons?
- roryokane 16y ago
  See my reply to someone else, describing the advantages and disadvantages of omitting semicolons: http://news.ycombinator.com/item?id=2184289 http://news.ycombinator.com/item?id=2184289 As for whether you should omit the semicolons, only you can decide how much those pros and cons affect you. Depending on how much you care about others who read your code, you might also want to consider how you think those other people would choose, if they evaluated those pros and cons for themselves.
drivebyacct2 16y ago
I wish they were. It can cause unexpected problems. http://bonsaiden.github.com/JavaScript-Garden/#semicolon http://bonsaiden.github.com/JavaScript-Garden/#semicolon (this really wasn't the example that I was thinking of, I'll scrounge for the other examples, sorry)
- jacobolus 16y ago
  That example has nothing to do with whether semicolons are omitted or not; the rule that you need to know is don’t ever put a newline between the “return” keyword and the value to be returned, but whether you add semicolons at the end of what you want to make a statement makes absolutely no difference in this case. In other words, whether you write return foo; or return foo the result is the same: an empty return, completely ignoring the foo bit.
  - drivebyacct2 16y ago
    Right, but if it wasn't "autoinserting" semicolons, the behavior would be different.
    - jacobolus 16y ago
      That doesn’t have any bearing on whether human programmers should add unnecessary semicolons to lines where there is no such ambiguity.
flyosity 16y ago
Good luck minifying/compressing your JavaScript files without using semicolons. There's nothing I hate more than finding a promising jQuery plugin and when I integrate it into our JS minifying processes it blows up because they left semicolons off of a few lines, or the whole thing.
- jamesjyu 16y ago
  I want to second that. I've had to contact numerous js library authors to warn them about missing semicolons which break minimization.
- deleted 16y ago
  [deleted]
- jacobolus 16y ago
  Which minifier blows up for you? Sounds like time for a bug report filed against the tool.
- roryokane 16y ago
  I’ve never personally minified JavaScript, with or without semicolons, but the article “Semicolons in JavaScript are optional” linked to in the page says the author minified jQuery without semicolons with Google Closure Compiler, and the test suite passed. Is that article wrong? Did minifying semicolons use to work, but they removed that feature for some reason? Does no one actually use Closure Compiler, and the other minifiers don’t support missing semicolons? Or is the reason the plugin you found “blows up” simply the error described in “Semicolons in JavaScript are optional” in the section “The only real pitfall when coding without semicolons”, which is starting a line with “(d + e)” instead of “;(d + e)”? That article says that if a minifier doesn’t understand JavaScript that an actual JavaScript interpreter can understand, the minifier is buggy. I agree with it – if your minifier messed up the semicolon-less jQuery plugin, and if none of the situations I mentioned above was the case, then your minifier needs fixing, or you should start using a different one.
- IsaacSchlueter 16y ago
  Unless you can name a minifier that actually breaks in this case, I'm gonna have to call BS on this.
paulirish 16y ago
Unless you get a perfect score, don't omit semicolons http://asi.qfox.nl http://asi.qfox.nl Oliver Hunt, primary JavascriptCore engineer took this quiz and said, "I rewrote our entire JS parser < 2 months ago, with all ASI rules and still didn't get 100% on a quiz"
- jacobolus 16y ago
  Most of those examples are designed to push the edge cases in confusing ways (i.e. bad code). Easy answer: don’t do that. Even if you include semicolons in all the “reasonable” places for them, it’s quite possible to get tripped up if you don’t understand what counts as a statement in JS and what doesn’t. Edit to add: For example, don’t write code like this, with or without the autoinserted semicolons: var x = function() { return (function(n){ return n })(n) }(function(){Math.exp( f )}) Someone else looking at the code is hardly going to be better off understanding it if you change that to: var x = function() { return (function(n){ return n; })(n); }(function(){Math.exp( f );});
- oomkiller 16y ago
  Do you write code like this? If so, you have bigger issues than ASI!
beej71 16y ago
I think ECMA-262-5 7.9 pretty concisely sums it up... and at the same time there are plenty of nasty examples in the "FUD" section at the bottom of the article. The spec very clearly says they may be used, so I trust no one will have a problem with me using them. It's tough to drop that old C habit, anyway. Even my Python code has a random scattering of semicolons at the ends of lines.
mustpax 16y ago
Omitting semicolons in JavaScript is like omitting braces from one line if blocks. It's all well and good until someone turns this: if (conditional) statement; Into this without realizing what they've just done: if (conditional) statement; statement2; Yes, you can skip semicolons. And yes it will work the vast majority of the time. But the amount of time you save will be eclipsed by that one time someone makes a change to the code that breaks in an unpredictable subtle way due to your purely aesthetic shorthand. Writing maintainable code is about learning humility. There's no need to show off your understanding of the deeper points of JavaScript semicolon insertion in your production code base. Just insert the semicolons, I promise you you will learn to not be distracted in no time.
- dionidium 16y ago
  I've never understood this argument for always including braces in one-line if-blocks. Won't this error be immediately obvious? I mean, after making the change, won't even the most minimal test catch the mistake? If developer #2 isn't doing the most basic test to verify that "statement2" runs when "conditional" is true, then you've got way bigger problems than your style guide.
  - mustpax 16y ago
    Anecdotally I've seen smart and capable developers make this mistake numerous times. It usually gets caught by code review, automation etc. But not always. It's basically the birthday paradox. You take the probability of a very unlikely event and raise it to a big enough power (enough people, enough lines of code) it becomes a non-negligible outcome.
  - mathgladiator 16y ago
    developer #2 probably doesn't care because it passes the unit tests and no one is the wiser. I've seen a lot of bugs during some consulting work that was simply the result of either intellectually laziness or un-disciplined coding.
  - gte910h 16y ago
    > Won't this error be immediately obvious? No, smart people miss it all the time.
tedunangst 16y ago
The semicolons are optional people really seem like they are picking a fight just to pick a fight. Why not explain how my JS will be better if I omit some semicolons and not just the same?
- roryokane 16y ago
  I would think they didn’t explain how it makes JS better because they were on the defensive from people who said leaving semicolons out will break code. They are saying “stop forbidding me from coding in the style I want, my style works just fine”. They are just trying to defend their style, not convert people to it. However, that’s not to say there is no reason to omit semicolons. I personally leave out semicolons as an extension of the DRY principle – if you are putting a line break after a statement anyway, then you might as well leave off the semicolon. It’s one less character to delete if you combine two adjacent lines into one. Also, if you already think of line breaks and statement ends going together, then semicolons are just noise that you have to make an effort to ignore – the effort required may be small, but there’s little reason not to avoid that effort. The only problem with omitting semicolons in JavaScript is this case mentioned in “Semicolons in JavaScript are optional”: a = b + c (d + e).print() The author recommends avoiding the error in that by changing the last line to ;(d + e).print() . If the extra effort of adding a semicolon before all line-starting parentheses costs more effort than you would save by getting the two advantages I described above, then maybe adding semicolons everywhere is right for you. You might have to experiment with coding in both styles to be able to accurately quantify how much those two advantages and one disadvantage would affect you.
serichsen 16y ago
The problem is not the semicolons, it is the newlines, and there is little you can do about that. The newline/semicolon issue is the most ridiculous language design decision ever.
__Joker 16y ago
Slightly out of topic https://groups.google.com/forum/?pli=1#!msg/nodejs/MWaivVTirPY/0pnRjKsggkIJ https://groups.google.com/forum/?pli=1#!msg/nodejs/MWaivVTir...

21 points by jashkenas 16y ago

deleted 16y ago

[deleted]

To the author of the website: you have modified google analytics code and removed semicolons, I congratulate you on that. It shows true conviction. :) Joke aside, that's a good content put together in very clear and concise manner.

jpadvo 16y ago

I always use semicolons to end lines in JavaScript because it is comfortable for me. ...and I always feel a little guilty about it, because it seems wrong to add superfluous characters to my code. Is it actually "personal preference" and I've been feeling guilty for nothing, or is it better to leave off the semicolons?

roryokane 16y ago
See my reply to someone else, describing the advantages and disadvantages of omitting semicolons: http://news.ycombinator.com/item?id=2184289 http://news.ycombinator.com/item?id=2184289 As for whether you should omit the semicolons, only you can decide how much those pros and cons affect you. Depending on how much you care about others who read your code, you might also want to consider how you think those other people would choose, if they evaluated those pros and cons for themselves.

drivebyacct2 16y ago

I wish they were. It can cause unexpected problems. http://bonsaiden.github.com/JavaScript-Garden/#semicolon http://bonsaiden.github.com/JavaScript-Garden/#semicolon (this really wasn't the example that I was thinking of, I'll scrounge for the other examples, sorry)

jacobolus 16y ago
That example has nothing to do with whether semicolons are omitted or not; the rule that you need to know is don’t ever put a newline between the “return” keyword and the value to be returned, but whether you add semicolons at the end of what you want to make a statement makes absolutely no difference in this case. In other words, whether you write return foo; or return foo the result is the same: an empty return, completely ignoring the foo bit.
- drivebyacct2 16y ago
  Right, but if it wasn't "autoinserting" semicolons, the behavior would be different.
  - jacobolus 16y ago
    That doesn’t have any bearing on whether human programmers should add unnecessary semicolons to lines where there is no such ambiguity.

flyosity 16y ago

Good luck minifying/compressing your JavaScript files without using semicolons. There's nothing I hate more than finding a promising jQuery plugin and when I integrate it into our JS minifying processes it blows up because they left semicolons off of a few lines, or the whole thing.

jamesjyu 16y ago
I want to second that. I've had to contact numerous js library authors to warn them about missing semicolons which break minimization.
deleted 16y ago
[deleted]
jacobolus 16y ago
Which minifier blows up for you? Sounds like time for a bug report filed against the tool.
roryokane 16y ago
I’ve never personally minified JavaScript, with or without semicolons, but the article “Semicolons in JavaScript are optional” linked to in the page says the author minified jQuery without semicolons with Google Closure Compiler, and the test suite passed. Is that article wrong? Did minifying semicolons use to work, but they removed that feature for some reason? Does no one actually use Closure Compiler, and the other minifiers don’t support missing semicolons? Or is the reason the plugin you found “blows up” simply the error described in “Semicolons in JavaScript are optional” in the section “The only real pitfall when coding without semicolons”, which is starting a line with “(d + e)” instead of “;(d + e)”? That article says that if a minifier doesn’t understand JavaScript that an actual JavaScript interpreter can understand, the minifier is buggy. I agree with it – if your minifier messed up the semicolon-less jQuery plugin, and if none of the situations I mentioned above was the case, then your minifier needs fixing, or you should start using a different one.
IsaacSchlueter 16y ago
Unless you can name a minifier that actually breaks in this case, I'm gonna have to call BS on this.

paulirish 16y ago

Unless you get a perfect score, don't omit semicolons http://asi.qfox.nl http://asi.qfox.nl Oliver Hunt, primary JavascriptCore engineer took this quiz and said, "I rewrote our entire JS parser < 2 months ago, with all ASI rules and still didn't get 100% on a quiz"

jacobolus 16y ago
Most of those examples are designed to push the edge cases in confusing ways (i.e. bad code). Easy answer: don’t do that. Even if you include semicolons in all the “reasonable” places for them, it’s quite possible to get tripped up if you don’t understand what counts as a statement in JS and what doesn’t. Edit to add: For example, don’t write code like this, with or without the autoinserted semicolons: var x = function() { return (function(n){ return n })(n) }(function(){Math.exp( f )}) Someone else looking at the code is hardly going to be better off understanding it if you change that to: var x = function() { return (function(n){ return n; })(n); }(function(){Math.exp( f );});
oomkiller 16y ago
Do you write code like this? If so, you have bigger issues than ASI!

beej71 16y ago

I think ECMA-262-5 7.9 pretty concisely sums it up... and at the same time there are plenty of nasty examples in the "FUD" section at the bottom of the article. The spec very clearly says they may be used, so I trust no one will have a problem with me using them. It's tough to drop that old C habit, anyway. Even my Python code has a random scattering of semicolons at the ends of lines.

mustpax 16y ago

Omitting semicolons in JavaScript is like omitting braces from one line if blocks. It's all well and good until someone turns this: if (conditional) statement; Into this without realizing what they've just done: if (conditional) statement; statement2; Yes, you can skip semicolons. And yes it will work the vast majority of the time. But the amount of time you save will be eclipsed by that one time someone makes a change to the code that breaks in an unpredictable subtle way due to your purely aesthetic shorthand. Writing maintainable code is about learning humility. There's no need to show off your understanding of the deeper points of JavaScript semicolon insertion in your production code base. Just insert the semicolons, I promise you you will learn to not be distracted in no time.

dionidium 16y ago
I've never understood this argument for always including braces in one-line if-blocks. Won't this error be immediately obvious? I mean, after making the change, won't even the most minimal test catch the mistake? If developer #2 isn't doing the most basic test to verify that "statement2" runs when "conditional" is true, then you've got way bigger problems than your style guide.
- mustpax 16y ago
  Anecdotally I've seen smart and capable developers make this mistake numerous times. It usually gets caught by code review, automation etc. But not always. It's basically the birthday paradox. You take the probability of a very unlikely event and raise it to a big enough power (enough people, enough lines of code) it becomes a non-negligible outcome.
- mathgladiator 16y ago
  developer #2 probably doesn't care because it passes the unit tests and no one is the wiser. I've seen a lot of bugs during some consulting work that was simply the result of either intellectually laziness or un-disciplined coding.
- gte910h 16y ago
  > Won't this error be immediately obvious? No, smart people miss it all the time.

tedunangst 16y ago

The semicolons are optional people really seem like they are picking a fight just to pick a fight. Why not explain how my JS will be better if I omit some semicolons and not just the same?

roryokane 16y ago
I would think they didn’t explain how it makes JS better because they were on the defensive from people who said leaving semicolons out will break code. They are saying “stop forbidding me from coding in the style I want, my style works just fine”. They are just trying to defend their style, not convert people to it. However, that’s not to say there is no reason to omit semicolons. I personally leave out semicolons as an extension of the DRY principle – if you are putting a line break after a statement anyway, then you might as well leave off the semicolon. It’s one less character to delete if you combine two adjacent lines into one. Also, if you already think of line breaks and statement ends going together, then semicolons are just noise that you have to make an effort to ignore – the effort required may be small, but there’s little reason not to avoid that effort. The only problem with omitting semicolons in JavaScript is this case mentioned in “Semicolons in JavaScript are optional”: a = b + c (d + e).print() The author recommends avoiding the error in that by changing the last line to ;(d + e).print() . If the extra effort of adding a semicolon before all line-starting parentheses costs more effort than you would save by getting the two advantages I described above, then maybe adding semicolons everywhere is right for you. You might have to experiment with coding in both styles to be able to accurately quantify how much those two advantages and one disadvantage would affect you.

serichsen 16y ago

The problem is not the semicolons, it is the newlines, and there is little you can do about that. The newline/semicolon issue is the most ridiculous language design decision ever.

__Joker 16y ago

Slightly out of topic https://groups.google.com/forum/?pli=1#!msg/nodejs/MWaivVTirPY/0pnRjKsggkIJ https://groups.google.com/forum/?pli=1#!msg/nodejs/MWaivVTir...